1. Field of the Invention
The present invention generally concerns magnetic resonance tomography (MRT) as used in medicine for examination of patients. The present invention in particular concerns methods for accelerated and improved (with regard to the signal-to-noise ratio (SNR) image generation from raw data that are or have been acquired with a type of technique known as the PROPELLER technique. A number of reception coils are necessary in order to use this technique.
2. Description of the Prior Art
The conventional PROPELLER technique is a radial sampling (scanning) method in MRT imaging and is briefly described as follows. A more detailed presentation can be found in the article of the magazine Magnetic Resonance in Medicine 42: 963-969 (1999) by James G. Pipe. In the PROPELLER technique, sampling of k-space ensues on the basis of a series of blades. Each of the blades is composed of L equidistant, parallel phase coding lines. A blade thus contains the L lines of a conventional k-space trajectory with Cartesian sampling for which the phase coding gradient has the smallest amplitude. The k-space sampling according to the PROPELLER technique is dependent on the manner that the individual blades of the series are rotated relative to one another around the center of k-space. The rotation angle αi and the number N of the total number of blades are characteristic parameters that are selected such that the series covers or overlaps the entire k-space of interest (see FIG. 2). A significant feature of the PROPELLER k-space trajectory is that a circular region (with diameter L) in the center of k-space is covered by every single blade. As a result, in comparison to other acquisition methods in MRT, the PROPELLER technique is extremely robust with regard to patient movement during the data acquisition. The comparison of two different blades with regard to this center data enables movements of the patient in the image plane that occur between the acquisition of two blades to be determined. This movement can then be corrected in the framework of the PROPELLER image reconstruction. The comparison of the center data also enables the identification of such blades that can not/could not be movement-corrected (in particular given patient movement out of the image plane) and exclusion thereof from the image reconstruction.
The assumption forming the basis of the PROPELLER movement correction is that each blade is separately sampled quickly with regard to typical patient movements, such that any movement is frozen.
It is known that an acceleration of the method (the data acquisition) in MRT can generally be achieved by PPA (partial parallel acquisition). In Cartesian imaging, data acquisition with PPA methods enables one or more k-space lines to be excluded in the phase coding direction and for the missing information (the missing lines) to be algebraically reconstructed afterwards from the measured lines. A number of reception coils (for example a multi-component coil array of surface coils) are a requirement for this, and the respective spatial sensitivity of each coil must be known. The spatial sensitivity depends on, among other things, the orientation of the coil on the patient and the respective load in the receptive field (this depends on the patient himself or herself).
The determination of the coil sensitivities (or generally the determination of data known as coil calibration data) is therefore a component of every PPA technique.
The omission of lines does in fact lead to a significant reduction of the acquisition time, but at the cost of a notable SNR decrease and thus a lower image quality.
In the technical literature a number of different PPA methods are described. Peter Kellman, “Parallel Imaging: The Basics”, ISMRM Educational Course: MR Physics for Physicists, 2004 gives an overview. A specific PPA method that is applied in the embodiment of the present invention is GRAPPA. GRAPPA was first described in the journal article by Mark A. Griswold, Peter M. Jakob, Robin M. Hidemann, Mathias Nittka, Vladimir Jellus, Jianmin Wang, Berthold Kiefer, Axel Haase, “Generalized Autocalibrating Partially Parallel Acquisitions (GRAPPA)”, Magn. Reson. Med., 47: 1202-1210, 2002. The embodiment of the present invention also currently supports further developments of GRAPPA. These developments of GRAPPA are, for example, described by E. G. Kholmovski, A. A. Samsonov in “GARSE: Generalized Autocalibrating Reconstruction for Sensitivity Encoded MRI”, ISMRM 2005, abstract #2672. In the work (cited above) by E. G. Kholmovski, A. A. Samsonov, the further-developed GRAPPA method is given the new name “GARSE”.
The acquisition time of PROPELLER MRI can be significantly shortened by PPA methods (for example see James G. Pipe, “The use of Parallel Imaging with PROPELLER DWI”, ISMRM 2003, abstract #66.
Common to all previous PPA-PROPELLER implementations is the selection of the blade width (the number of the lines in a blade), which corresponds to A times the width of a conventional scan and such that only data in each A-th line are acquired during the data acquisition (see FIGS. 3 and 4); each blade thus falls below the Nyquist criterion by the A-multiple. A is thereby generally designated as an acceleration factor. In order to avoid aliasing as a result of the under-sampling of the respective blade, a PPA reconstruction is individually applied to each blade before the actual PROPELLER reconstruction. The actual PROPELLER reconstruction then corresponds to the reconstruction method from conventional (non-accelerated) PROPELLER and is described in the journal article by James Pipe (cited previously). It typically includes the steps of phase correction, rotation correction, displacement correction, correlation weighting, interpolation on a Cartesian grid and final Fourier transformation. Optionally, some of these steps can be omitted (for example the correlation weighting).
Two different techniques for determination of the coil sensitivities required for PPA reconstruction are specified in the literature:
The first technique is to estimate the coil sensitivities on the basis of a conventional PROPELLER scan separately applied for this. This technique is applied, for example, by T-C. Chuang, T-Y. Huang, F-H. Linear, F-N. Wang, H-W. Chung, C-Y. Chen, K. Wong in “Propeller EPI with SENSE parallel imaging using a circularly symmetric phase array RF coil”, ISMRM 2004, abstract #535.
The second technique includes the estimation of the coil sensitivities on the basis of the central k-space zone (common to all blades) with diameter L. In order to obtain the coil calibration data for the PPA reconstruction of a specific blade, the central data of all blades are interpolated on the grid of this blade (using what is known as a gridding algorithm). This method is based on the assumption that the central k-space zone is sampled by all blades according to the Nyquist criterion. This assumption is correct only for acceleration factors that are not too large. If no patient movement occurs during the measurement and if the requirement cited for the second technique is fulfilled, both techniques are suitable for the determination of the coil sensitivities.
If a significant movement of the patient ensues during the actual measurement, however, the relation of the coil calibration data to the respective blades is lost since the PPA reconstruction occurs before a movement correction (rotation, displacement).
In such a case, measurement data of different blades are intermixed by the PPA reconstruction method and movement artifacts are thereby generated, and in fact also when each individual blade is separately measured quickly relative to typical patient movement. This is a significant, severe disadvantage in comparison to the conventional PROPELLER imaging in MRT and stands in contradiction to the fundamental assumption that each blade is essentially free of any movement effects.
It should be noted that it is not possible to implement a movement correction before the PPA reconstruction, since a movement registration cannot be applied to under-sampled blades.